I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a method for assigning optimal packet lengths in a variable rate communication system.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in a system with a large number of users. Although other techniques such as time division multiple access (TDMA), frequency division multiple access (FDMA), and AM modulation schemes such as amplitude companded single sideband (ACSSB) are known, CDMA has significant advantages over these other techniques. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS," and assigned to the assignee of the present invention and incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", assigned to the assignee of the present invention and incorporated by reference herein. The CDMA system can be designed to conform to the "TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System", hereinafter referred to as the IS-95 standard. Another code division multiple access communication system includes the GLOBALSTAR communication system for world wide communication utilizing low earth orbiting satellites.
CDMA communication systems are capable of transmitting traffic data and voice data over the forward and reverse links. A method for transmitting traffic data in code channel frames of fixed size is described in detail in U.S. Pat. No. 5,504,773, entitled "METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION", assigned to the assignee of the present invention and incorporated by reference herein. In accordance with the IS-95 standard, the traffic data or voice data is partitioned into code channel frames which are 20 msec in duration. The data rate of each code channel frame is variable and can be as high as 14.4 Kbps.
A significant difference between voice services and data services is that the former requires a fixed and common grade of service (GOS) for all users. Typically, for digital systems providing voice services, this translates into a fixed (and guaranteed) data rate for all users and a maximum tolerable value for the error rates of the speech frames, independent of the link resource. For the same data rate, a higher allocation of resource is required for users having weaker links. This results in an inefficient use of the available resource. In contrast, for data services, the GOS can be different from user to user and can be a parameter optimized to increase the overall efficiency of the data communication system. The GOS of a data communication system is typically defined as the total delay incurred in the transfer of a data message.
Another significant difference between voice services and data services is the fact that the former imposes stringent and fixed delay requirements. Typically, the overall one-way delay of speech frames must be less than 100 msec. In contrast, the data delay can become a variable parameter used to optimize the efficiency of the data communication system.
The parameters which measure the quality and effectiveness of a data communication system are the total delay required to transfer a data packet and the average throughput rate of the system. Total delay does not have the same impact in data communication as it does for voice communication, but it is an important metric for measuring the quality of the data communication system. The average throughput rate is a measure of the efficiency of the data transmission capability of the communication system.
Given the growing demand for wireless data applications, the need for very efficient wireless data communication systems has become increasingly significant. An exemplary communication system which is optimized for data transmission is described in detail in copending U.S. patent application Ser. No. 08/963,386, entitled "METHOD AND APPARATUS FOR HIGH RATE PACKET DATA TRANSMISSION", filed Nov. 3, 1997, assigned to the assignee of the present invention, and incorporated by reference herein. The system disclosed in U.S. patent application Ser. No. 08/963,386 is a variable rate communication system capable of transmitting at one of a plurality of data rates. The data rate is selected in accordance with the link between the transmitting base station and the receiving remote station. The transmitting base station is selected from among all base stations in communication with the remote station based on the links from the base stations to the remote station. The selected base station transmits to the remote station for a predetermined duration of time which is referred to as a time slot. Since the data rate is known and the time slot is fixed, the packet length (or the number of transmitted bits within the time slot) can be computed.
Transmitting from the best base station at all times to the highest rate user results in the highest throughput rate possible in a variable rate communication system. However, this scheme suffers from lack of fairness, in the sense that all but the highest data-rate users get a very poor quality of service. A policy that is both fair and efficient favors the higher rate users by awarding them a larger number of contiguous time slots as compared to lower data rate users, but ensures that even the lowest data rate users get a reasonable share of the system throughput. As stated above, a parameter which measures the quality of the communication system is the total delay experienced by all users. A method is required to assign the packet length, or the time slot, for each data rate such that the throughput rate is maximized while maintaining a level of fairness.